1. Field of the Invention
This work was supported by the IT R&D program of MIC/IITA. [2005-S-051-02, Photonic device integrated module for optical access network]
The present invention relates to a semiconductor optical amplifier (SOA), and more particularly, to a reflective SOA (R-SOA) and a superluminescent diode (SLD) having low operating and threshold currents.
2. Description of the Related Art
An optical line terminal (OLT) and an optical network unit (ONU) used in a Wavelength Division Multiplexing Passive Optical Network (WDM-PON) have several types of light sources. According to a recently announced paper in IEEE PTL, vol. 18, no. 13, pp. 1436-1438, an OLT using a distributed feedback laser diode (DFB-LD), which oscillates in a single mode, and an ONU using a reflective-semiconductor optical amplifier (R-SOA), which depends on the polarization of the light, have been used to perform two-way communications of 1.25 Gbp/s or more.
A high-priced DFB-LD matching with a WDM channel is required to be replaced with a low-priced light source in order to lower a price of a WDM-PON. External cavity lasers (ECL), which can be manufactured at low cost, attract attention due to the above requirement. In such an ECL, a multi-channel light source can be manufactured on a single substrate. Thus, lots of research into ECLs has been conducted. An ECL has a structure in which a grating is formed at a silica waveguide, and a semiconductor laser as a light source is hybrid integrated. The ECL must not oscillate so that reflectivity of an emission surface of the semiconductor laser as the light source of the ECL is 0.1% or less. Also, the ECL must be a device having a high gain at a low operating current. Fabry-Perot laser diodes (FP-LD) and superluminescent diodes (SLD) are available as light sources satisfying above-described conditions.
An anti-reflection (AR) coating of 0.1% or less is formed on a surface to reduce reflectivity of an emission surface of the FP-LD. Also, a high-reflection (HR) coating is formed on an opposite surface to obtain a high gain. For example, the Japanese NTT group manufactures multi-channel ECLs using FP-LD on which an AR coating and an HR coating are formed. However, in general, it is very difficult to form an AR coating of only 0.1% or less on an emission surface of a FP-LD in order to prevent oscillation. Also, yield is low.
A superluminescent diode (SLD) is a light source which can replace the AR coated FP-LD. An active layer or an optical waveguide is inclined at an angle of 7° or 10° in a general SLD to reduce reflectivity of an emission surface of the SLD. In this case, the reflectivity of the emission surface can be reduced, but the SLD is not suitable as a light source used for a WDM-PON due to a high threshold current and operating current. Thus, the SLD requires a light source having the same characteristic as an FP-LD on which an AR coating and an HR coating are formed.
Also, an R-SOA used in an ONU simply amplifies and modulates non-interferential light allocated to each subscriber. Thus, the R-SOA reduces noise due to a gain saturation characteristic without greatly changing output power in spite of a slight change of a spectrum according to external conditions. However, such an R-SOA requires a light source consuming a small amount of power at low threshold and operating currents to be used in a WDM-PON.